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Simplify executor's determination of whether to use parallelism.

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Our parallel-mode code only works when we are executing a query
in full, so ExecutePlan must disable parallel mode when it is
asked to do partial execution.  The previous logic for this
involved passing down a flag (variously named execute_once or
run_once) from callers of ExecutorRun or PortalRun.  This is
overcomplicated, and unsurprisingly some of the callers didn't
get it right, since it requires keeping state that not all of
them have handy; not to mention that the requirements for it were
undocumented.  That led to assertion failures in some corner
cases.  The only state we really need for this is the existing
QueryDesc.already_executed flag, so let's just put all the
responsibility in ExecutePlan.  (It could have been done in
ExecutorRun too, leading to a slightly shorter patch -- but if
there's ever more than one caller of ExecutePlan, it seems better
to have this logic in the subroutine than the callers.)

This makes those ExecutorRun/PortalRun parameters unnecessary.
In master it seems okay to just remove them, returning the
API for those functions to what it was before parallelism.
Such an API break is clearly not okay in stable branches,
but for them we can just leave the parameters in place after
documenting that they do nothing.

Per report from Yugo Nagata, who also reviewed and tested
this patch.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/20241206062549.710dc01cf91224809dd6c0e1@sraoss.co.jp
This commit is contained in:
Tom Lane
2024-12-09 14:38:19 -05:00
parent a0ab20f165
commit 4089b9bd6f
5 changed files with 29 additions and 39 deletions
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4
src/backend/tcop/postgres.c
View File

@@ -1254,7 +1254,7 @@ exec_simple_query(const char *query_string)
(void) PortalRun(portal,
FETCH_ALL,
true, /* always top level */
true,
true, /* ignored */
receiver,
receiver,
&qc);
@@ -2230,7 +2230,7 @@ exec_execute_message(const char *portal_name, long max_rows)
completed = PortalRun(portal,
max_rows,
true, /* always top level */
!execute_is_fetch && max_rows == FETCH_ALL,
true, /* ignored */
receiver,
receiver,
&qc);

13
src/backend/tcop/pquery.c
View File

@@ -670,6 +670,8 @@ PortalSetResultFormat(Portal portal, int nFormats, int16 *formats)
* isTopLevel: true if query is being executed at backend "top level"
* (that is, directly from a client command message)
*
* run_once: ignored, present only to avoid an API break in stable branches.
*
* dest: where to send output of primary (canSetTag) query
*
* altdest: where to send output of non-primary queries
@@ -714,10 +716,6 @@ PortalRun(Portal portal, long count, bool isTopLevel, bool run_once,
*/
MarkPortalActive(portal);
/* Set run_once flag. Shouldn't be clear if previously set. */
Assert(!portal->run_once || run_once);
portal->run_once = run_once;
/*
* Set up global portal context pointers.
*
@@ -922,7 +920,7 @@ PortalRunSelect(Portal portal,
{
PushActiveSnapshot(queryDesc->snapshot);
ExecutorRun(queryDesc, direction, (uint64) count,
portal->run_once);
false);
nprocessed = queryDesc->estate->es_processed;
PopActiveSnapshot();
}
@@ -962,7 +960,7 @@ PortalRunSelect(Portal portal,
{
PushActiveSnapshot(queryDesc->snapshot);
ExecutorRun(queryDesc, direction, (uint64) count,
portal->run_once);
false);
nprocessed = queryDesc->estate->es_processed;
PopActiveSnapshot();
}
@@ -1406,9 +1404,6 @@ PortalRunFetch(Portal portal,
*/
MarkPortalActive(portal);
/* If supporting FETCH, portal can't be run-once. */
Assert(!portal->run_once);
/*
* Set up global portal context pointers.
*/